The present invention relates to clutches and particularly automotive clutches.
The present invention relates more particularly to automotive clutches of the kind comprising, in axial succession, a reaction plate or flywheel adapted to be secured by fastening means to a first shaft which in practice is the driving shaft, a clutch disc or plate adapted to be fixed for rotation with a second shaft which in practice is a driven shaft, a pressure plate fixed for rotation with the reaction plate though axially movable with respect to the reaction plate, a cover axially fixed to the reaction plate, and rotatable therewith, and resilient means bearing against the cover and constantly biasing the pressure plate toward the reaction plate for axially clamping or squeezing the clutch plate between the pressure plate and the reaction plate.
In this kind of clutch the operation of which is well known, the entire clutch assembly is rotatable, and outside relatively brief periods when the driving and driven shafts are operatively disconnected by axially releasing the pressure plate by deformation of the resilient means, rotates at the speed of rotation of the driving shaft. In present-day internal combustion engines this speed commonly exceeds 100 r.p.m. or 315 radians per second. The corresponding centrifugal acceleration is 100 g at 1 centimeter from the axis. It is clear that taking into account that the entire clutch assembly rotates, the centering and the balancing of the clutch assembly are extremely important for the proper operation and reliability of clutch assembly.
Moreover, the entire active mechanism of the clutch is located in an enclosed space defined between the reaction plate and the cover. Owing to the location of the clutch plate between the reaction plate and the pressure plate and the fixing of the pressure plate for rotation with the driven shaft, the fixing of the pressure plate for rotation with and axial movement with respect to the reaction plate is necessarily effected along the outer periphery beyond the outer diameter of the clutch plate.
Such a clutch is assembled on a jig comprising two coaxial mandrels axially displaceable with respect to each other. One mandrel receives the reaction plate and the other the cover. The clutch plate and the resilient means such as a diaphragm spring are mounted on centering bridges while the pressure plate is centered from the exterior. Thereafter the mandrels are moved toward each other to fix the cover on the reaction plate, thereby preloading the resilient means. The securement of the cover on the reaction plate and the axially mobile connection between the pressure plate and the reaction plate are determinative for the balancing of the clutch. Nevertheless, it is generally necessary to refine the balance by controlled local removal of stock.
It is evident that the general reference axis of the clutch is the axis of the driving shaft. However, this reference is not available at the time the clutch is assembled. Also, the reaction plate undergoes a close tolerance machining operation for coupling, which defines a secondary axial reference for the assembly of the clutch, all the components being centered with respect to this secondary reference.
Conventionally the cover is assembled with the reaction plate by connecting an outer radial rim of the cover on a faced surface of the reaction plate by fastening means such as screws. Now, the cover which is generally dish-shaped with a central opening is stamped from a sheet metal blank. The rim is edged and its surface that comes into contact with the reaction plate is faced. Yet these operations, like the mounting on the mandrel of the assembly jig, are executed with the cover gripped in a chuck jaw, whose outer surface is substantially as it comes from the stamping operation. Because of finishing operations the centering tolerances are necessarily large. The influence of such large centering tolerances on the cover for the balance of the assembled clutch is not serious, and is easily compensated for by the final balancing operation. But it is out of the question to use the cover as a reference for the centering of the other components of the clutch.
Now, the rationalization of the structure of the clutch or the adaptation of the clutch to a particular type of engine may make it desirable to fix such other components on the cover. In particular when the pressure plate is fixed for rotation with the reaction plate by straps fixed to lugs on the pressure plate which protrude through cutouts in the sidewall of the cover, which straps are anchored on spacer members riveted to the cover. Thus the centering of the pressure plate is maintained by the straps for which the cover does not provide a centering reference.